1. Technical Field
The present invention relates to semiconductor devices, and also relates to methods for manufacturing the same.
2. Related Art
Ferroelectric memory devices (FeRAM) are nonvolatile memories capable of low voltage and high speed operations, and their memory cells can be each formed from one transistor and one capacitor (1T/1C). Accordingly, ferroelectric memory devices can achieve integration at the same level of that of DRAM, and are therefore expected as large-capacity nonvolatile memories.
As the structures of such ferroelectric memory devices, a planer type (see, for example, Japanese laid-open patent application JP-A-2003-347512) and a stacked type (see, for example, Japanese laid-open patent application JP-A-2006-310637) may be enumerated. A ferroelectric memory device in either of the structures is equipped with a ferroelectric capacitor having a ferroelectric film placed between a pair of electrodes. One of the pair of electrodes is connected to a bit line through a transistor, and the other electrode is connected to a ground line or the like. Generally, they are electrically connected through plugs that may be composed of tungsten or the like.
The ferroelectric film described above may be formed from a ferroelectric material having a perovskite type crystal structure represented by a general formula ABO3, and more specifically, may be composed of lead zirconate titanate (Pb (Zi, Ti) O3: PZT) or the like. The ferroelectric material is an oxide, and therefore needs care so as not to be reduced and thus deteriorated.
Therefore, according to the aforementioned patent documents, a hydrogen barrier film that covers the ferroelectric capacitor is formed, such that the ferroelectric film would not be reduced when exposed to a reducing atmosphere in later steps. According to the document JP-A-2003-347512, when forming plugs on ferroelectric capacitors to be connected to the plugs, a titanium nitride film (barrier metal) having hydrogen barrier property is formed inside contact holes in which the plugs are formed, whereby the ferroelectric film can be prevented from being reduced even when the plugs are formed in a reducing atmosphere.
However, if the measures are not sufficient against the reducing atmosphere at the time of forming the plugs over the ferroelectric capacitor in the methods described in the aforementioned patent documents, the ferroelectric film may possibly be deteriorated. Generally, an interlayer dielectric film is formed to a sufficient thickness, and then polished and thinned by a CMP method or the like, to obtain a desired thickness. However, due to differences in the thickness of the film at the time of film formation and the polishing amount caused by unevenness in the base layer, the interlayer dielectric film may have differences in its thickness. For this reason, when forming contact holes by etching the interlayer dielectric film and the hydrogen barrier film, the differences in the thickness of the interlayer dielectric film are reflected in the hydrogen barrier film, which makes it difficult to uniformly etch the hydrogen barrier film.
As a result, the side wall of the hydrogen barrier film in the contact hole cannot be formed into a desired shape. When a titanium nitride film (barrier metal) is formed, as described in the document JP-A-2003-347512, weak points are created in the film, such that the titanium nitride film cannot be functioned satisfactorily. For example, the hydrogen barrier film is excessively etched in thinner portions of the interlayer dielectric film, and the side wall of the hydrogen barrier film forms a shape that rises perpendicularly with respect to the upper electrode, as shown in FIG. 1 of the document JP-A-2003-347512. Therefore, it is difficult to fill the barrier metal material well into corners at the stepped sections, which present weak points.
When such weak points are formed, a reducing gas may penetrate the ferroelectric capacitor through the weak points at the time of forming the plugs, thereby reducing the ferroelectric film and causing oxygen deficiencies and deterioration. Memory cells with the deteriorated ferroelectric films cannot provide good hysteresis characteristic, and therefore present bit failures, whereby the bit yield is deteriorated.